This invention relates to the production of phosphate fertilizers and, more particularly, to the production of diammonium phosphate and "merchant" phosphoric acid.
Fertilizer manufacturers in the United States have established a standard analysis for diammonium phosphate of 18% nitrogen and 46% phosphoric oxide (18-46-0). Similarly, merchant phosphoric acid has a standard analysis of 52 to 54% phosphoric oxide and a solids content of less than 2% by weight when shipped. These standards have allowed diammonium phosphate and merchant phosphoric acid to become fungible products. For the fertilizer manufacturer, there is a need to maintain these standards in order to utilize trade associations in export of these products and to maintain acceptance of products by the trade.
The analysis of diammonium phosphate and the solids content and physical properties of merchant phosphoric acid depend on the impurities content of the phosphoric acid used to make the products. The impurities in turn derive from the phosphate rock used to make phosphoric acid; they remain in the phosphoric acid when gypsum is filtered from the solution formed by the reaction of phosphate rock and sulfuric acid in the various "wet acid" processes used to make fertilizer phosphoric acid.
In addition to precluding attainment of the standard analysis for diammonium phosphate of 18-46-0, excessive levels of impurities also increase solids precipitation when phosphoric acid from the gypsum filtration step is concentrated. Such impurities cause "post-precipitation", the precipitation of solids after shipping. Post-precipitated solids are difficult to remove from shipping and storage vessels, increase the apparent viscosity of the merchant acid making the material more difficult to handle, and decrease the value of the product.
Numerous approaches have been tried to reduce post-precipitation in merchant phosphoric acid; none has met with wide success.
Solids precipitating when filtered acid is concentrated and cooled have been removed by sedimentation or other liquid-solid separations. Solids removed from acid containing about 40% phosphoric oxide (P.sub.2 O.sub.5) are relatively low in P.sub.2 O.sub.5 and may be discarded without undue economic loss. However, solids separated as "sludge" from phosphoric acid containing 52 to 54% P.sub.2 O.sub.5 as in the production of merchant phosphoric acid, contain so much P.sub.2 O.sub.5 that they cannot be discarded without undue economic loss. Because of the high impurity content of sludge, however, its use such as in the production of phosphate fertilizers is limited. Limited use for the sludge, in turn, has previously limited the fraction of merchant acid that can be produced from wet process phosphoric acid.
Reduction of impurities by precipitating and separating them as solids is disclosed in U.S. Pat. No. 4,376,756. Super phosphoric acid, 70 to 72% P.sub.2 O.sub.5, and 10-34-0 with reduced tendencies to precipitate solids in storage are produced from mixtures of treated and untreated acids. At least 20% and, typically, 25% or more of the combined acid must be treated to achieve a useful improvement. Further, the examples show a calculated removal of only 7.9 to 9.8% of MgO, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 (as equivalents of cations per equivalent P.sub.2 O.sub.5) from a mixture of treated and untreated dihydrate phosphoric acids at the stated minimum ratio and the minimum typical ratio of treated to untreated acid, respectively.
Other precipitation, filtration and similar schemes are reported in U.S. Pat. Nos. 2,891,856; 3,467,162; 3,684,438; 3,890,097; 3,993,735; 4,236,911 and Chemical Abstracts references 78P 32,163g; 80P 38,864z; 81P 65,709e; 83P 12,906b; 83P 62,886r; and 90P 206,663k.
In addition to reducing impurities by physical separation of solids, a variety of processes have been suggested for reducing impurity levels in merchant phosphoric acid. Dolomite can be removed from phosphate rock by means of heavy media separation and flotation to reduce magnesium impurity levels in the starting materials for acid production. This treatment is applicable only to certain phosphate ores, however. See in this regard, Lawver et al. "Beneficiation of South Florida Carbonate Phosphorites", ISMA Technical Conference, October 1978.
Cation removal by ion exchange has also been suggested to reduce impurity levels in phosphoric acid. See, for example, U.S. Pat. No. 4,280,904. The limited capacity of strong acid exchange resins and the relatively high concentration of cationic impurities in wet process phosphoric acid require a large volume of resin in relation to acid volume; the ratio of resin volume to acid can approach unity. Because of this high volume of resin, losses of phosphoric acid are high and the products suffer from dilution in washing the loaded resin. Moreover, regeneration acid for regenerating the resin is needed. In practice, about two equivalents of regeneration acid are consumed per equivalent of cation impurity removed. Accordingly, high costs attend this approach. See also U.S. Pat. No. 3,993,735.
Solvent extraction has been used to purify phosphoric acid. Typically, these processes have been operated to produce a relatively pure phosphoric acid for industrial chemical use. One process described in U.S. Pat. No. 3,867,511 produces a partially purified phosphoric acid for liquid fertilizer use. The impurities removed from the extracted phosphoric acid, however, are concentrated, for example, as raffinate phosphoric acid. The quantity of P.sub.2 O.sub.5 in the raffinate is too large--30 to 50% of the P.sub.2 O.sub.5 fed to the process--to be discarded. In this respect, the solvent extraction purification described in U.S. Pat. No. 3,867,511 is like separation of precipitated solids from 52 to 54% P.sub.2 O.sub.5 phosphoric acid. An improved product is obtained, but a co-product stream containing a high level of impurities is produced. The impure co-product must be commercially utilized for efficient operation of the phosphate fertilizer plant.
Other methods for purification of phosphoric acid through solvent separation are reported in U.S. Pat. Nos. 3,388,967; 3,953,581; and 4,018,869.
Still other approaches to acid purification are related in U.S. Pat. Nos. 1,648,146; 3,141,734; 3,644,091; 3,907,680; 4,110,422; 4,248,846; 4,048,289; and 4,250,154 together with Chemical Abstracts 78 161,564n; 83P 134,385b; 86P 31,584y; 91P 23,359s; 92 61,065w and 94 159,080p.